The present invention relates generally to wireless communication systems, and more particularly to wireless communication systems that use channel estimation in closed loop transmit diversity modes.
In wireless communication systems (e.g., cellular systems) operating in a closed loop mode, increased network capacity or performance, i.e., the ability to carry more data between a base station and a mobile user unit, means increased profitability. When, for instance, a code division multiple access (CDMA) communication system operating in a closed loop mode transmits a communication from a base station over a transmission channel termed a traffic channel, several distortions including transmission and signal capture-related distortions may occur that can constrain any significant improvements in the network capacity or performance. Moreover, in a typical close loop mode, multiple active mobile user units may simultaneously access the traffic channel. In addition, a feedback channel from one or more targeted mobile user units of the multiple active mobile user units may be used to transmit feedback information to the base station. However, oftentimes signals in many wireless communication systems experience a variety of transmission and signal capture-related distortions such as fading and multipath interference. Therefore, the feedback channel may encounter some degree of fading and multipath interference as well. In some situations, this may result in a finite time delay in the communication from the one or more targeted mobile user units to the base station.
One technique of improving network capacity and performance is to reduce the finite time delay involved while transmitting and capturing feedback information over a feedback channel (e.g., a radio link) between the one or more targeted mobile user units and the base station. However, the delay that needs to be compensated, is mostly caused by the time needed to transmit the feedback information over the feedback channel. Thus, under these circumstances, a compensation for the finite time delay may be difficult to accomplish.
Specifically, to obviate fading of a transmission channel (e.g., the traffic and/or the feedback channels) in a closed loop mode where a mobile user unit may be moving slowly, a type of feedback power control may be used. However, when fading rate increases, as is the case with the mobile user unit moving at relatively higher speeds, a closed loop power control becomes practically ineffective. For overcoming multipath interference of a transmission channel, in a closed loop mode, channel estimation is typically performed as the multiple active mobile user units are prone to multipath fading, i.e., the traffic channel consists of more than one distinct propagation path for each mobile user unit of the multiple active mobile user units. Channel estimation typically includes determining channel parameters to compensate for certain transmission and signal capture-related distortions.
In order to perform channel estimation at the mobile station, the base station may transmit a pilot channel having one or more pilot symbols to the multiple active mobile user units for providing appropriate timing and other information. And, in turn, a targeted mobile user unit may acquire desired channel parameters that may be formed from the pilot channel using the pilot symbols. Examples of the pilot symbols include a pseudo noise (PN) code, a spreading sequence, or a scrambling sequence for the multiple active mobile user units seeking a base station with which to affiliate. However, channel estimation is generally limited only to detection and makes numerous limiting assumptions, such as an assumption regarding timing information for the traffic channel.
A variety of diversity modes have been widely used in closed loop modes in several wireless communication systems, as they purportedly provide better signal qualities in fading channel environments while significantly increasing network capacity and performance. A diversity mode essentially entails transmitting or receiving a signal with at least two antennas where at least two different parameters including location or polarization are used. A diversity mode may be applied to transmit diversity or receiver diversity. In transmit diversity, “N” number of transmit antennas at a base station may be employed when “N” number of fading channels may need to be estimated in a mobile user unit making use of known pilot symbols from a pilot channel.
For instance, a CDMA communication system may use dual transmit diversity (i.e., two transmit and one receive antenna) for estimating multipath fading channels. However, at least in part due to a significantly long time delay involved with feedback in a closed loop, there may be lack of knowledge available concerning instantaneous channel states (e.g., timing and attenuation) for each multipath fading channel. In absence of precise channel state knowledge, therefore, it may not be possible to accurately estimate the channel parameters to adequately compensate for certain transmission and signal capture-related distortions.
Unfortunately, combination of feedback with channel estimation alone, in closed loop transmit diversity modes, may not provide adequate compensation to the antenna transmission because the channel estimation may not keep up with a quickly changing transmission channel between a base station and a mobile user unit that may be moving. Therefore, while communicating using transmit diversity in a closed loop mode between a base station to a mobile user unit, an improved compensation technique for transmission over a channel is desired accordingly.
Thus, there is a need to accurately estimate transmission patterns in wireless communication systems while operating in closed loop transmit diversity modes.